1. Field of the Invention
The present invention relates to a technique for forming a resonance part using an inductance element and a variable capacitance element, and a voltage controlled oscillator (VCO) using the resonance part.
2. Description of the Related Art
As shown in FIG. 13, for instance, a voltage controlled oscillator (VCO) includes a resonance part having a varicap diode VD whose electrostatic capacitance varies in accordance with a control voltage and an inductance element 11, and a feedback part 2 formed of a transistor 21 being an amplifying part and two capacitors C1, C2. In this example, a frequency signal resonated in the resonance part is amplified by the transistor 21, and is fed back to a series resonant circuit via the feedback part 2, thereby forming an oscillation loop. Note that 31 in FIG. 13 denotes a buffer amplifier that amplifies the frequency signal and outputs it to the outside. Further, 16, T3 and L denote an input terminal, an output terminal part and an inductance element, respectively. Although the illustration is omitted, the VCO is disposed on a base substrate made of ceramics such as LTCC (Low Temperature Co-fired Ceramics) using alumina (Al2O3) as a main component, for instance.
Here, if the VCO is used in a high frequency band of, for example, several GHz or several tens of GHz by increasing an oscillation frequency thereof, problems as follows occur. Specifically, in a high frequency band, the VCO may become a distributed constant circuit in which a size of the substrate becomes larger than a wavelength of electrical signal processed (output) by the above-described VCO, and in such a case, there is a possibility such that signals whose amplitudes are reversed flow on the substrate, and these signals interfere with each other, which results in outputting no electrical signal, or the size of the substrate including the VCO has to be reduced to a size with which a practical manufacturing becomes difficult.
For instance, the VCO is disposed on a not-shown base substrate made of ceramics such as LTCC using alumina (Al2O3) as a main component. The LTCC has a relative dielectric constant ∈ of about 9 to 10, for example, so that an apparent wavelength of electrical signal propagating on the substrate becomes shorter than an actual wavelength. Accordingly, in order to suppress the interference of electrical signals, it is preferable to reduce a size of the substrate to, for example, about 1/10 of the wavelength of the electrical signal, but, in reality, it is difficult to form electric circuits or is mount electronic components on a substrate with such a size.
Furthermore, in a VCO mounted in a radio communication device that outputs an electrical signal with a quite high frequency, which is, for example, an electrical signal in a GHz band (microwave), it is conceivable that a) an output signal of the VCO outputting a signal having a frequency lower than the frequency band is multiplied, b) GaAs (gallium arsenide compound) or the like is used as a base substrate, and c) a cavity resonator is used. However, in the case of a), a phase noise becomes large since a multiplication circuit is used. Further, in the case of b), the cost is increased, and in the case of c), the size is large so that it becomes difficult to realize the downsizing.
Although Patent Document 1 describes a device such as the above-described VCO, no study has been made regarding the aforementioned problems. Further, according to Patent Document 2, there is known a technique for making a quartz crystal being a piezoelectric substrate generate an elastic wave to use the quartz crystal as a resonator whose oscillation frequency is in about several MHz band, for instance, but, it is not possible to solve the above-described problems.
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 10-209714
[Patent Document 2] Japanese Patent Application Laid-open No. 2007-201772